Effective treatment of internal combustion engine emissions can be provided through catalytic treatment of such emissions by a catalyst disposed in a conventional catalytic converter. The catalyst temperature must be elevated above light-off to support such activity. The heat energy present in engine exhaust gas provides for such heating. Following an engine coldstart, a typical catalytic converter in a vehicle underfloor position (underfloor converter) may require more than sixty seconds of energy transfer from the engine exhaust gas to reach light-off. Prior to reaching light-off, the effectiveness of the converter is substantially limited.
To provide for some catalytic treatment of the engine exhaust gas prior to the time the conventional underfloor converter reaches light-off, small warm-up "pup" catalytic converters have been proposed. Such pup converters may be positioned in an internal combustion engine exhaust gas path very close to the engine exhaust manifold, to minimize loss of heat energy between the engine and the converter and thus to maximize heat energy transfer to the pup converter. The pup converter and the catalyst contained therein are both of significantly smaller volume than the conventional converter. The location and size of the pup converter thus contribute to a significantly shorter light-off time than that of conventional converters. Following light-off, the pup may contribute some catalytic treatment to the engine exhaust gas, to reduce vehicle emissions following a coldstart of the engine. After the conventional catalytic converter reaches light-off, it takes on the majority of exhaust gas catalytic treatment.
To sustain an acceptable degree of catalytic treatment performance, it has been proposed to place a conventional oxygen sensing device, traditionally used for closed-loop engine air/fuel ratio control, in the engine exhaust gas path in a position downstream of the conventional catalytic converter, to monitor and report the oxygen storage capacity of the converter. In the event such capacity is determined to be diminished, indicating that the converter may not be adequately treating engine exhaust gas, the vehicle operator may be notified so that the converter may be repaired or replaced.
The diagnostic approach using an oxygen sensor downstream of the conventional catalytic converter may be extended to monitor the pup converter by placing an additional conventional oxygen sensor between the pup converter and the conventional catalytic converter to monitor the oxygen storage capacity of the pup converter. In its more traditional automotive role, the oxygen sensor would also be placed upstream of the both the pup and the conventional converters to monitor the oxygen content in the engine exhaust gas as a measure of the actual engine air/fuel ratio. An output signal from this upstream sensor would be available for comparison to a desired engine air/fuel ratio and a controlled correction to commanded air or fuel to the engine made in response to the comparison.
Limitations arise in a sensor arrangement in which three oxygen sensors are provided in the engine exhaust gas path including a first sensor between the engine and the pup converter for engine air/fuel ratio control, a second sensor between the pup and conventional converter for pup converter diagnostics, and a third sensor downstream of the conventional converter for converter diagnostics. Specifically, the first sensor output voltage will oscillate between a rich and lean voltage responsive to normal closed-loop engine control activity. The second sensor output voltage will, due to the oxygen storage capacity of a "healthy" pup converter, oscillate very little, remaining at a substantially steady value. The third sensor output voltage will likewise appear as a substantially steady value with very little oscillation. While the inactivity of the second signal indicates normal oxygen storage capacity in the pup converter, and thus may be used for comparison to the first signal to diagnose the pup converter, the insignificant change in sensor output signal between the second and third sensors provides virtually no measure of the oxygen storage capacity of the conventional converter.
Furthermore, it is desirable to locate the pup converter as close to the engine as possible to maximize heat transfer to the catalyst thereof. Such proximity to the engine leaves little room for locating an oxygen sensor between the pup converter and the engine for traditional engine air/fuel ratio control. Furthermore, if room is found for location of that oxygen sensor upstream of the pup converter, it may be positioned so as to provide a poor indication of overall engine air/fuel ratio. For example, if the sensor is located in or closer to an exhaust manifold of one bank of an engine having a plurality of cylinder banks, or is located so close to the engine that proper mixing of individual cylinder exhaust has not yet occurred, the accuracy of the sensor in indicating overall engine air/fuel ratio may be decreased, leading to loss of air/fuel ratio control accuracy.
It would therefore be desirable to provide for monitoring of both the pup and conventional catalytic converters while providing for precise engine air/fuel ratio control.